Plants also suffer from summer sunburn.
During hot days you should take care not only of your balcony plants but also of your indoor plants. Both can get sunburned if they stay in the blazing sun too long. The sun exploits humidity from the plants which they need to live and grow. This is noticeable by brown spots or by dry leaves in spite of sufficient watering.
To avoid this, you should keep your house plants away from southern windows. You also can lower shutters slightly to shed them from strong sun rays during midday.
Balcony plants which react sensitive to strong sun should be moved into the shade. Still, sufficient watering is important and should not be forgotten.